Plastisols in which polymer fine particles are dispersed in a plasticizer as a medium have been used in various fields of industry such as automotive undercoats, auto body sealers, wall papers, carpet backings, floor materials, paints, and toys.
Conventionally, plastisols have mostly been vinyl chloride sols using vinyl chloride polymer particles. In recent years, a shift to (meth)acrylic plastisols using (meth)acrylic polymer particles (hereinafter, expressed as “acrylic sols”) has been investigated from the consideration of global environment and the like. The acrylic sols do not generate toxic dioxins or halogenated hydrogen gases that are a factor of acid rain when products produced from the acrylic sols are incinerated because the acrylic sols do not contain halogen atoms.
The shift from vinyl chloride sols to the acrylic sols is being positively advanced, in particular, in the uses of automotive undercoats and auto body sealers. The reason of this is, in addition to the above environmental problem, to exclude vinyl chloride sols that generate hydrogen chloride and the like that cause damage of equipment at a melting step of shredder dust at the time of recycling automotives.
Generally, in the uses of automotive undercoats and auto body sealers, adhesives are compounded to plastisol compositions for the purpose of giving adhesibility because it is required for a coating film to adhere strongly and closely to a body base material. However, the adhesives are highly viscous liquid materials, and hence, stringiness or sagging tends to occur and thereby workability tends to be lowered at the time of coating the plastisols.
Further, there is a tendency that material costs of the acrylic sols become higher as compared with those of vinyl chloride sols. As a main cause of this, the fact that raw material cost of (meth)acrylic polymer particles tends to be raised and adhesives, curing agents, and anti-sagging agents tend to be compounded in great quantities can be listed.
Although various proposals have been offered to solve the problems of occurrence of stringiness or sagging at the time of coating acrylic sols and costs of compounding materials, the most effective measure is the curtailment of the adhesives.
Methods for curtailing the adhesives are proposed, for example, in Patent Document 1 and Patent Document 2. In Patent Document 1, a technology for giving acrylic sols excellent adhesibility by copolymerizing a heterocyclic compound having at least one nitrogen atom with a (meth)acrylic polymer particle is proposed. Further, in Patent Document 2, a technology for improving storage stability of acrylic sols and giving acrylic sols adhesibility by adding and polymerizing the whole or the most part of a monomer having a basic nitrogen atom after at least half of another copolymerizable monomer is polymerized has been proposed.
However, in both technologies, storage stabilities are insufficient at a temperature of 30 to 40° C. or above, which is an industrial operating environment, though they are excellent at a room temperature.
In Patent Document 3, a plastisol composition containing a plasticizer and a particulate polymer mixture composed of at least two components A and B is described.
In the case of using the technology of Patent Document 2 or 3, there is a tendency that a resin having excellent storage stability is insufficient in adhesive strength, while a resin having excellent adhesive strength is insufficient in storage stability, and hence, it is difficult to keep the balance between both the performances. As the reason of this, when the polymer have excellent compatibility with a plasticizer, it is presumed that a polymer having basic nitrogen atoms which is an adhesive component become easily movable to the interface of a resin and a base material at the time of baking, so that adhesibility tends to be improved, at the same time, the polymer easily absorb the plasticizer at the time of storage, so that viscosity change of acrylic sols become large and storage stability tend to become insufficient. On the other hand, it is also presumed that, in the case that the polymer is incompatible with the plasticizer, the viscosity change of acrylic sols become small owing to the difficulty of absorption of the plasticizer by the polymer at the time of storage, however, the polymer become hard to move to the interface of the resin and the base material at the time of baking, so that adhesibility tend to be insufficient.
An example of using a monomer having a blocked isocyanate as a functional group is proposed in Patent Document 4, however, the balance between storage stability and adhesive strength is not kept, which is same as in the above-mentioned case.
Patent Document 1: Japanese Patent Application Laid-Open No. 51-71344
Patent Document 2: Japanese Patent Application Laid-Open No. 52-42590
Patent Document 3: Japanese Patent Application Laid-Open No. 2004-27233
Patent Document 4: Japanese Patent Application Laid-Open No. 2006-299006